1. Field of the Invention
The present invention generally relates to a circuit board structure and a method for fabricating the same, and more specifically, to a fine-pitch circuit board structure and a method for fabricating the same.
2. Description of Related Art
A conventional substrate structure is illustrated in FIGS. 1A and 1B, which are cross-sectional views of a substrate taken from different directions. Solder pads 11 of the conventional substrate for electrically connecting an electronic element are generally SMD (solder mask defined) solder pads, that is, exposed size of the solder pads 11 depends on size of the openings defined by the solder mask layer 12, such that when solder balls are wetted to the solder pads 11, height of the solder balls after collapse can be controlled by the solder mask layer 12. The solder mask layer 12 is adapted to protect conductive traces 13 of the substrate from being damaged by external forces or contaminated by pollutants, and further prevent the conductive traces 13 from getting in contact with the solder balls disposed on the solder pads 11 which may cause short circuit.
However, actual size of the solder pads 11 must be larger than that of the openings of the solder mask layer 12, that is, area of the substrate actually occupied by the solder pads 11 is larger than area of the solder pads 11 exposed from the openings of the solder mask layer 12, and width of the solder pads 11 is apparently larger than width of the conductive traces 13. As a result, the number of the conductive traces 13 that are allowed to pass between the solder pads 11 is quite limited, thus making it difficult to meet a desired design requirement.
For example, conventional SMD solder pads have a size of 800 μm, size of the solder pads exposed from the openings of the solder mask layer is 600 μm, and pitch x between adjacent solder pads is 1270 μm, width of conductive traces is about 100 μm, it can be seen that the SMD structure would only allow a pair of conductive traces to pass through, pitch between adjacent conductive traces is about 90 μm. Obviously, the number of conductive traces that are allowed to pass between the conventional SMD solder pads is quite limited.
In the case a conductive trace 13 cannot pass between the solder pads 11 in a same plane, a build-up technique is applied to form a dielectric layer 15 such that the conductive trace 13 can be connected to another layer of the substrate through a conductive blind via 14 formed in the dielectric layer 15 so as to form a solder pad 11, which however adversely increases the number of the substrate layer and thus increases the fabrication cost and complicates the fabrication process.
In an attempt to solve the problem described above, as depicted in FIG. 2, U.S. Pat. No. 5,706,178 discloses an oval-shaped solder pad structure 21 to increase the routing area between adjacent solder pads 11 and further allow more conductive traces 23 to pass through. Moreover, U.S. Pat. Nos. 6,396,707 and 6,543,128 disclose an elongate-shaped solder pad structure 31, wherein an opening 220 is formed in a solder mask layer 22 to expose partial area of the solder pad 31 as well as two sides of the solder pad 31, such that more conductive traces are allowed to pass between the solder pads through variation in the shape of the solder pads.
For instance, if the short-axle width of the oval-shaped solder pad 21 is 600 μm and width of the elongate-shaped solder pad 31 is smaller than 600 μm, the opening 220 adapted to expose the solder pad from the solder mask layer is 600 μm, the pitch between the centers of two adjacent solder pads is 1270 μm, and width of the conductive traces 23 is about 100 μm, then about four conductive traces can pass through the pitch of about 54 μm between two adjacent solder pads. Although more conductive can pass between adjacent solder pads through variation in shape of solder pads, the available routing area between adjacent solder pads are still constrained by pitch between the openings of the solder mask layer, that is, the number of the conductive traces that can pass between adjacent solder pads depends on and is limited by pitch between the openings. Consequently, this approach still fails to provide effective routing area between adjacent solder pads so as to allow more conductive traces to pass through.
Moreover, if width of the elongate-shaped solder pads or size of the openings of the solder mask layer are too small, wetting area of solder balls may become insufficient, thereby adversely affecting strength of solder joints and height after collapse of solder balls.
As such, it is highly beneficial and desirable to come up with a new approach that can effectively increase the number of conductive traces passing through pitch between solder pads while being able to eliminate drawback of insufficient solder ball wetting area caused by too narrow width of solder pads and adversely affecting strength of solder joints and height after collapse of solder balls.